It's something a child would dream of ( he had it since he was like 4). And knowing Luffy something not very articulate. The big party or something like that is the most credible I heard but it wouldn't explain why some of the crew were startled when they first heard it, but that may just be Oda trying to mistify it. Maybe it's that everyone's dream will become reality (which is also how the Devil's Fruit were born). I think it would something so big and so small at the same time it's ridiculous. Man, I love One Piece ahahah